I have a plaster fireplace (ornamental only)... I can move it like a piece of furniture.

It used to have a raised carving in the middle of the face(?) of what looks to be leaves, flowers and ribbons. The outline of this design is still there and slightly raised. I'd like to build it up again but am unsure of what material is best, and how I would make sure that it adhered to the surface of the fireplace.

I thought of using modeling clay....but would it be too heavy and how would I attach it? (and then there's the problem of learning to carve...hmmm)

Also, thought of using plaster (redi-crete) and building it up layer by layer. (again with the problem of carving).

Welcome Dancer:
I would look around for a wood carving in antique stores, at cabinet shops, reclaimed building materials stores or the internet. A good construction adhesive should hold it in place.
Glenn